(1) Field of the Invention
The present invention relates to an image forming apparatus which directly forms the image on recording medium such as recording paper etc., by causing developer particles to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
In recent years, as the image forming means for outputting a visual image on recording medium such as recording paper etc., in response to an image signal, an image forming apparatus is disclosed in Japanese Patent Application Laid-Open Hei 6 No.155,798, for example, in which developer particles, i.e., toner, are made to directly adhere to the recording medium to thereby form a toner image on it, directly.
Referring to FIGS. 1 and 2, a printer of the configuration represented by the image forming apparatus defined in Japanese Patent Application Laid-Open Hei 6 No.155,798 will be described. This apparatus includes an image forming unit 51 having a toner supplying section 52 and a printing section 53. In this apparatus, toner 71 carried in toner supplying section 52 is selectively made to jump to and adhere to a sheet-like recording paper 55 as a recording medium. During this operation, the jumping of toner 71 is controlled in accordance with an image signal so that the toner can selectively adhere to recording paper 55 directly, forming a visual image.
Toner supplying section 52 is composed of a toner reservoir 70 for holding toner 71 as developer particles which are, for example, negatively charged, and a toner support 72 for supporting toner 71 using, for example, magnetic force. Toner support 72 is grounded and rotationally driven in the direction indicated by arrow E in the figure, with its surface speed set at 30 mm/sec, for example. Toner 71 is of a magnetic type having a mean particle diameter of 10 .mu.m, and is electrified with static charge of -4 .mu.C/g to -5 .mu.C/g by a well-known technique. Toner 71 is carried on the peripheral surface of toner support 72 with a mean thickness of about 80 .mu.m.
As to these, referring first to the conventional apparatus configured as shown in FIG. 1, printing section 53 as a part of image forming unit 51 is composed of an opposing electrode 75 made up of an aluminum pipe of, for example, 50 mm in diameter, and a control electrode 76 which is provided between opposing electrode 75 and a toner support 72. Opposing electrode 75 is arranged about 1 mm apart from the peripheral surface of toner support 72, has a high voltage, e.g., 2 kV applied from a d.c. power source 80, and is rotationally driven in the direction of arrow F in FIG. 1, with its surface speed set at 30 mm/sec. Therefore, an electric field is generated between opposing electrode 75 and toner support 72 that causes toner 71 supported on toner support 72 to jump toward opposing electrode 75.
Control electrode 76 is disposed in parallel to a tangent plane of the surface of opposing electrode 75 and spreads two-dimensionally facing opposing electrode 75, and it has a structure which permits the toner to pass therethrough from toner support 72 to opposing electrode 75. The electric field formed between toner support 72 and opposing electrode 75 varies depending on the potential being applied to control electrode 76, so that the jumping of toner 71 from toner support 72 to opposing electrode 75 is controlled.
Control electrode 76 is arranged so that its distance from the peripheral surface of toner support 72 is set at 100 .mu.m, for example. Control electrode 76 is composed of a flexible print board (FPC) 76a 50 .mu.m thick and annular electrodes 77 . . . of a copper foil of 20 .mu.m thick. Board 76a has gates 79 . . . having a diameter of 150 .mu.m for passage of toner 71. Around these gates 79 . . . are arranged the aforementioned annular electrodes 77 . . . . Each annular electrode 77 is electrically connected via a feeder line and high-voltage driver (neither is unillustrated) to a control power source 81.
Voltages are applied to electrodes 77 . . . from control power source 81, corresponding to the image signal. Detailedly, when toner 71 supported on toner support 72 is made to travel toward opposing electrode 75, control power source 81 applies a voltage, e.g., 200 V to annular electrodes 77 . . . , so that toner 71 can jump through gates 79 of annular electrodes 77 to the recording paper on the opposing electrode side. In contrast, if the toner need not be passed, the power source 81 applies -200 V to annular electrode 77 to prohibit the toner on toner support 72 from jumping toward opposing electrode 75. In this way, the application of voltages to annular electrodes 77 is performed in accordance with the image signal, so that it is possible to directly form a visual image corresponding to the image signal, on the recording paper 55, by selectively causing the toner to jump.
Here, the rotation of toner support 72, the rotation of opposing electrode 75, the application of voltage to control electrode 76 to prohibit passage of toner 71, and application of the high voltage to the opposing electrode are activated at almost the same time by a common trigger. The transfer time of toner 71 from toner support 72 to recording paper 55 is determined depending upon the amount of static charge on the toner, the distance, and the potential difference applied, between toner support 72 and opposing electrode 75, and in particular, depends on the intensity of the electric field. This time is about 250 .mu.sec, for example. The voltage application time to annular electrode 77 is set longer than the transfer time, specifically at about 300 .mu.sec. Thus, the toner is ensured to adhere to recording paper 55 on opposing electrode 75.
Referring to the conventional apparatus configured as shown in FIG. 2, printing section 53 as a part of image forming unit 51 is composed of an opposing electrode 75 made up of aluminum of about 2 mm thick, and a control electrode 76 which is provided between opposing electrode 75 and toner support 72. Opposing electrode 75 is arranged about 1 mm apart from the peripheral surface of toner support 72 and has a high voltage, e.g., 2 kV applied from a d.c. power source 80. Therefore, an electric field generated between opposing electrode 75 and toner support 72 that causes toner 71 supported on toner support 72 to jump toward opposing electrode 75.
Arranged over opposing electrode 75 is a dielectric belt 73 in contact therewith so that the belt can travel. Dielectric belt 73 is formed endless and is tensioned by at least two support rollers 73a and 73b. When support roller 73b is made to operate, the belt travels in the direction of arrow F. In order for dielectric belt 73 to electrostatically attract recording paper 55 and convey it, a charge supplying roller 74, which abuts dielectric belt 73 with recording paper 55 in between, is arranged downstream with respect to the conveyed direction of recording paper 55. Further, a power source 82 is connected to charge supplying roller 74 in order to cause recording paper 55 to be attracted to dielectric belt 73 by imparting a predetermined amount of charge.
Control electrode 76 is disposed parallel to the tangent plane of the surface of opposing electrode 75 and spreads two-dimensionally facing opposing electrode 75, and it has a structure to permit the toner to flow therethrough from toner support 72 to opposing electrode 75. The electric field formed between toner support 72 and opposing electrode 75 varies depending on the potential being applied to control electrode 76, so that the jumping of toner 71 from toner support 72 to opposing electrode 75 is selectively controlled.
When a high voltage, e.g., 1 kV is applied to charge supplying roller 74 by power source 82, negative charges are injected into recording paper 55 because support roller 73a is applied with 2 kV as stated above. Accordingly, the paper is electrostatically attracted to, and adheres to, dielectric belt 73 so that it can be conveyed to printing section 53 with the travel speed of dielectric belt 73.
Other configurations and operations of the apparatus of FIG. 2 is the same as those of FIG. 1.
These conventional apparatuses suffer from the following problems.
First, in the image forming apparatus of FIG. 1, a high voltage begins to be applied to opposing electrode 73 simultaneously with the application of voltage to other electrodes and the start of rotation of toner support 72. In this case, after a prolonged passage of time has elapsed from the deactivation of the image forming apparatus, or when toner support 72 just starts rotating, the surface of toner support 72 does not have the desired amount of charge and also the thickness of the toner layer supported thereon is not uniform either, so that most toner 71 exhibits unstable performance.
In such an unstable condition, when a high voltage is applied to opposing electrode 75 in order to perform the image forming operation, some toner 71 starts to transfer from toner support 72 to opposing electrode 75 passing through gates 79 even if the voltage which does not cause toner 71 to jump is being applied. This causes the problems that the surface of opposing electrode 75 is stained with toner 71 and that the underside of recording paper 55 being conveyed is stained by the transferred toner on the opposing electrode.
Some of the unstable toner 71 may jump to control electrode 76 due to its potential difference relative to opposing electrode 75 even if a voltage (-200 V in the conventional configuration) not causing toner 71 to jump is applied to control electrode 76. In this way, the adherence of toner 71 to control electrode 76 causes the apparent potentials of annular electrodes 77 . . . to vary due to the electric charge on the toner thus adhering, causing difficulty in the normal passage control of toner. Further, gate 79 for passage of toner might be clogged due to the toner particles adhering to control electrode 76, causing printing to be disabled in the worst case.
Of the unstable toner 71, a considerable amount of toner is reversely charged or has opposite polarity to the desired polarity. If there is such reversely charged toner 71, it adheres to control electrode 76 even if a voltage which will not cause toner 71 to jump (-200 V in the conventional configuration) is being applied. When the reversely charged toner 71 adheres to control electrode 76, the apparent potential of annular electrode 77 varies due to the charge on toner 71 as stated above, thus making it difficult to perform normal toner control. Further, this may make gate 79 clog or cause other troubles, followed by printing deficiency in the worst case.
In the image forming apparatus of FIG. 2, the high voltages applied to control electrode 76 and to opposing electrode are shut off simultaneously as soon as toner support 72 is deactivated at the end of the image recording operation.
Here, since opposing electrode 75 and control electrode 76 stated above have high voltages applied, toner 71 carried on toner support 72 is affected by the electric field created by the high voltages even after the cutoff because of the decay time constant. Since, in particular, a higher voltage is applied to opposing electrode 75 than to control electrode 76, if the supply of voltage to opposing electrode 75 is shut off, the electric field which causes the toner to jump toward opposing electrode will continue to exist because of the decay time constant. Therefore, some of toner 71 carried on toner support starts to transfer toward opposing electrode 75 passing through gates 79.
For the above reason, at the end of image forming, unnecessary toner tends to jump and adhere to opposing electrode 75 thus staining opposing electrode 75 with the toner. In order to prevent such phenomena, if image forming is temporarily stopped, a voltage which prohibits the toner from jumping toward the control electrode or opposing electrode, etc., may be adapted to be applied constantly. However, the maintenance of this voltage during the non-operating state of image forming, only causes waste of energy. Therefore, when image forming finishes, supply of voltage to the electrodes is usually stopped in time with the end of operation.
If the supply of voltage to opposing electrode 75 as well as to control electrode 76 is stopped at the end of image forming, the toner jumps to opposing electrode as stated above, staining the electrode with toner. If the image forming operation is restarted in this condition, the underside of recording paper 55 delivered will be stained disadvantageously.
The supply of the high voltage to opposing electrode 75, in particular, power circuit 80 etc., is not one in which supply and cutoff of high voltage is controlled by means of a relay circuit etc., but one in which the cutoff operation is effected by deactivating the oscillation of the voltage generator, specifically, DC--DC converter etc., to stop the generation of high voltage itself. This feature lengthens the decay time constant further, thus promoting the aforementioned problem of transfer and adherence of unnecessary toner to opposing electrode 75 after the end of image forming.
Of toner 71, some goes to opposing electrode 75 and some adheres to control electrode 76. If the image forming operation is restarted in this condition, the toner 71 adhering to control electrode 76 causes the apparent potentials of annular electrodes 77 . . . to vary due to the charge carried on toner 71, thus making it difficult to perform normal passage control of toner. Further, gate 79 for passage of toner might be clogged due to the toner particles adhering to control electrode 76, causing printing to be disabled in the worst case.
Even though toner 71 does not block gates 79 of control electrode 76, it adheres to the periphery of the gates 79 to make their openings narrower. If the image forming operation is restarted in this condition, the toner adhering to the interior of gates 79 disturbs jumping toner 71 traveling along the desired transfer path so that the toner cannot reach the predetermined dot forming area on recording paper 55. If toner 71 passes through gate 79 but does not reach the predetermined area on recording paper 55, the toner becomes scattered causing fogginess. That is, not only is it impossible to form a normal image but also toner 71 arriving on recording paper 55 causes the image to become blurred, forming a dim image without contrast. This especially causes difficulty in reproducing a halftone image or color image.
Moreover, the image forming apparatus with toner 71 thus adhering to control electrode 76 and opposing electrode 75 at the end of the image forming operation, is often left as it is for a long period of time. For example, suppose that the machine is left until the next day after the end of the image forming operation. Since toner 71 has a property that it clumps more as it is left longer, it becomes difficult to remove the toner which has adhered to control electrode 76 and was left. Once this happened, it is impossible to remove toner 71 by electrostatic force; it is necessary to detach control electrode 76 from printing section 51 and clean and remove it by force using an air brush or a typical cleaning brush. This task is very troublesome, and further brings about difficulty in attachment and adjustment of it to printing section 51 after cleaning.